1. Field of the Invention
This invention relates to an optical recording medium using a formazan metal complex dye in a recording layer and a method for photo-stabilizing a dye.
2. Prior Art
In recent years, optical recording discs of the write-once, rewritable or erasable and other types have been of great interest as high capacity information carrying media. Some optical recording discs use a dye film composed mainly of a dye as their recording layer. From a structural aspect, commonly used optical recording discs are classified into an air-sandwich structure type having an air space on a dye base recording layer and a close contact type having a reflective layer in close contact with a dye base recording layer. The latter has the advantage of enabling readout in accordance with the compact disc (CD) standard. For instance reference is made to Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and Proceedings SPIE--The International Society for Optical Engineering, Vol. 1078, pp. 80-87, "Optical Data Storage Topical Meeting", 17-19, January 1989, Los Angels.
The dyes used in such recording layers must be chemically stable and resistant to light. Since the recording layer is generally formed by spin coating from the manufacturing and cost points of view, the dyes must also have a sufficient solubility in certain solvents to form a coating solution. With respect to these requirements, phthalocyanine dyes are believed preferable for chemical stability and light resistance. However, the phthalocyanine dyes are substantially insoluble in most organic solvents and thus impractical unless they are subject to a solubilizing treatment as by introducing a substituent to convert into a soluble structure. They also have the disadvantages of a less variable wavelength range and an increased cost of manufacture.
Several metal complex dyes are known to meet the requirements of chemical stability and light resistance. For example, azo metal complex dyes-are disclosed in Japanese Patent Application Kokai (JP-A) No. 268994/1991 and bidentate formazan nickel complex dyes are disclosed in JP-A 254038/1985 and 144997/1987. The metal complex dyes are fully resistant to light, but generally have low recording sensitivity and relatively low solubility. They are soluble only in selected solvents. Another problem is crystallization occurring in a spin coated film. The bidentate formazan nickel complex dyes have a somewhat longer absorption wavelength and are deemed rather impractical when the wavelength (780 nm) of a semiconductor laser currently utilized for recording and readout of optical discs is taken into account. Since it is essential to shorten the wavelength of laser light for the advanced higher density recording technology under development, there is a strong demand to have a dye skeleton capable of meeting such a requirement too.
As is well known in the art, cyanine dyes are widely used in recording layers. The cyanine dyes are advantageous with respect to optical properties, solubility and manufacture cost, but extremely less resistant to light because of the sensitization action of singlet oxygen. As a general rule, photo-stabilizers are combined therewith. Typical photo-stabilizers are metal complexes such as nickel complexes of bis(phenylenedithiol). These metal complexes function as singlet oxygen quenchers while degradation of the complexes themselves and low solubility due to their plane configuration are problems. Also undesirably, their preparation is accompanied by disgusting smell and an increased cost.